detecting intra fraction motion in patients undergoing radiation treatment using a low cost wireless accelerometer

Received: 31 May 2009; in revised form: 1 August 2009 / Accepted: 18 August 2009 / Published: 27 August 2009 Abstract: The utility of a novel, high-precision, non-intrusive, wireless,

ISSN 1424-8220

www.mdpi.com/journal/sensors

Article

Detecting Intra-Fraction Motion in Patients Undergoing

Radiation Treatment Using a Low-Cost Wireless Accelerometer

Farid Farahmand 1 , Kevin O Khadivi 2 and Joel J P C Rodrigues 3, *

1

Department of Engineering Science, Sonoma State University, Rohnert Park, CA 94928, USA; E-Mail: farid.farahmand@sonoma.edu

2

Austin Cancer Center, 2600 E Martin Luther King Jr Blvd., Austin, Texas 78702, USA;

E-Mail: bepish@flash.net

3

Instituto de Telecomunicações, Department of Informatics, University of Beira Interior, Covilhã, 6201-001, Portugal

* Author to whom correspondence should be addressed; E-Mail: joeljr@ieee.org;

Tel.: +351-275-319-891; Fax: +351-275-319-888

Received: 31 May 2009; in revised form: 1 August 2009 / Accepted: 18 August 2009 /

Published: 27 August 2009

Abstract: The utility of a novel, high-precision, non-intrusive, wireless,

accelerometer-based patient orientation monitoring system (APOMS) in determining orientation change

in patients undergoing radiation treatment is reported here Using this system a small wireless accelerometer sensor is placed on a patient’s skin, broadcasting its orientation to the receiving station connected to a PC in the control area A threshold-based algorithm is developed to identify the exact amount of the patient’s head orientation change Through real-time measurements, an audible alarm can alert the radiation therapist if the user-defined orientation threshold is violated Our results indicate that, in spite of its low-cost and simplicity, the APOMS is highly sensitive and offers accurate measurements Furthermore, the APOMS is patient friendly, vendor neutral, and requires minimal user training The versatile architecture of the APOMS makes it potentially suitable for variety

of applications, including study of correlation between external and internal markers during Image-Guided Radiation Therapy (IGRT), with no major changes in hardware setup

or algorithm

Keywords: sensor networks; accelerometer; e-health; medical applications; medical

systems

1 Introduction

Advances in wireless sensor networks (WSNs) have created many new opportunities in healthcare and medical systems Examples of WSN applications in such areas are monitoring of blood pressure and oxygenation, breathing, heart rate, heart rhythm, electroencephalograms (EEGs) and electrocardiograms (ECGs) Wireless sensor networks have also been considered for high-precision, non-intrusive position monitoring

Patient position monitoring is particularly essential for accurately delivered radiation therapy treatments With the advent of Stereotactic Radiation Therapy (SRT) and ever decreasing target margins, even a nearly imperceptible movement may have a substantial negative impact on a patient’s outcome Vigilant active observation of patient movement serves two primary purposes: (1) it ensures that a patient who inadvertently moves during treatment will not receive an improperly directed dose; (2) it ensures that radiation is properly delivered, in spite of involuntary patient movements, e.g., breathing Advanced Image Guided Radiation Therapy (IGRT) techniques, such as Dynamic Targeting, often utilize these cyclical patient movements to theoretically improve treatment accuracy [1] Intrusive and/or cost-prohibitive methods are becoming less appealing as pain management throughout the treatment process gains more ground

Although considerable efforts are often undertaken to properly immobilize a patient, in many situations complete immobilization is not used or not possible Figure 1 depicts a typical setup to immobilize patient’s head rotation for the entire duration of the radiotherapy, which is highly uncomfortable for the patient Even when stringent immobilization precautions are taken, a patient’s head movements caused by physiological behaviours is generally extremely difficult, if not impossible,

to prevent [2,3] Similar studies have demonstrated that the problem is even more pronounced when patients are not thoroughly immobilized [4] Despite such difficulties, few accurate head position monitoring systems currently exist A passive visual monitoring by radiation therapists, using video cameras in the treatment room, is the prevalent conventional method More advanced options are currently available, yet these methods are generally expensive and cumbersome, with a somewhat limited function

In this preliminary study, we report our efforts in developing a wireless accelerometer-based patient orientation monitoring system (APOMS) The developed system is low-cost, low-power, small, and reusable and it has a highly flexible software platform The main focus of this system is to accurately detect head orientation changes due to involuntary patient’s movement The APOMS can potentially overcome many of the drawbacks associated with existing movement monitoring systems, such as setup complexity, high cost of maintenance and operation, or uncomfortable immobilization of the patient’s head as reported in [5] Instead of imaging and calculating the movement of a small fiducial positioned on a patient, as is commonly utilized [2], an APOMS can be directly attached to the patient’s head to report change in orientation without any imaging by ionizing radiation The accelerometer sensor in the APOMS has the capability to detect sub-degree rotation in all three orthogonal directions The number of sensors can easily be expended in order to simultaneously monitor undesired sudden orientation changes of multiple body parts Our test results indicate that in addition to being highly accurate and sensitive, the APOMS can perform well under high energy

radiation conditions Furthermore, while the APOMS provides sufficient signal strength for wireless communications, it does not interfere with radiation delivery

As a final note, it must be emphasized that an APOMS is not capable of detecting and measuring any gradual linear movement Thus, it cannot determine the actual amount of movement in the XYZ directions Our proposed system is limited to measuring orientation and changes in orientation and

detecting sudden positional changes A more sophisticated body positioning system should be used to

accurately measure movements in all six coordinates In this experiment, however, we are only concerned with head orientation changes, since the body is typically secured during radiotherapy

The remainder of this paper is structured as follow In Section 2, we describe a typical clinical treatment environment In Section 3, we elaborate on hardware description In Section 4, we present details of the signal processing algorithm In Section 5, we provide a general overview of the software implementation Finally, in Section 6, we characterize the performance of APOMS, followed by concluding remarks

Figure 1 A typical setup to avoid head orientation change during radiotherapy using

aquaplast thermoplastic (opti-mold) to provide pressure over the skin to immobilize head rotation

2 Clinical Treatment Environment

In this section we briefly describe the general setup used in a typical X-ray or radiation oncology (cancer therapy) procedure Figure 2(a) depicts a typical brain cancer therapy treatment The collimator, mounted on top of the gantry in the linear accelerator (linac), delivers radiation in a pre-determined pattern to the target The APOMS consists of two parts: the wireless node (WN) and the base station (BS) The accelerometer sensor of the wireless node is attached to the patient in the treatment room, as shown in Figure 2(a) The base station is placed in the control room for monitoring the patient’s head orientation As shown in Figure 2(a), the control room and the treatment room are completely isolated with a thick shielded wall The shielding structure has a very high attenuation factor and it is designed to prevent personnel access and limiting exposure to scattered radiation from

the collimator The total distance between the RF antenna of the WN and the BS is equivalent to d 1 +

d 2 + d 3

Figure 2(b) shows biodynamic coordinate system for human head (ISO-8727 defines the biodynamic coordinate system for human head In this paper we reference roll, pitch, and yaw to Z, X,

and Y coordinates, respectively, which are base on accelerometers’ coordinates) In this figure, one g

displacement (90 degree rotation) is equivalent to the gravitational attraction that the Earth exerts on objects (e.g., the head) or near its surface, and it is approximately 9.80665 m/s2 or 32.1740 ft/s2 As

shown in the figure, the accelerometer sensor and the radiation target are separated by distance R We

call this the exposure distance Location of the sensor and selection of R value must be such that any

change in orientation of the radiation target (patient’s head) can be accurately monitored and

measured In this figure, we assume the accelerometer sensor of the WN is attached to the chin of the

patient

Figure 2 (a) Radiation treatment setup using the linear accelerator (b) Head coordinates

(a) (b)

3 Hardware Description

Figure 3 depicts the main hardware components of APOMS prototype The small (4 × 4 × 1.45 mm)

accelerometer, mounted on a breadboard (2 × 2 cm), is attached to the patient’s body The wireless

communication between the accelerometer and the base station is supported by an Xbee transceiver

[6] Figure 3(a) shows the two main components of APOMS: WN and BS In the following paragraphs

we describe the functionality of each component in details

3.1 Wireless Node (WN)

The wireless node (WN) consists of two separate parts wired together: the accelerometer sensor and

the Xbee transceiver The sensor, shown in Figure 3(b), is a small, low-g, low-power iMEMS

accelerometer from Analog Devices (ADXL330) [7] An accelerometer can measure both dynamic and

static change of acceleration The device is a polysilicon surface micromachined structure built on top

of a silicon wafer [8] When the structure moves, the dynamic changes of acceleration in all directions

are detected using independent X, Y, and Z axes Each axis reports the recent magnitude of

acceleration using an analog voltage The voltage is converted to an equivalent g value, which can be

used to calculate the tilt angle Consequently, the choice of accelerometer was based on its ability to

achieve high degree resolution of tilt measurement and sensitivity within a measurement range

of (+/í) 3g

Figure 3 (a) Basic block diagram of APOMS; (b) ADXL330 accelerometer mounted on

the breakout board; (c) Xbee transceiver

Xbee RX

Linear Accelerator (LINAC)

Power

LabView Platform

USB

Alarm

Interruption

Xbee ADC

Xbee TX

X,Y,Z

Accelero meter

Power Supply

WN

BS

(a)

The outputs of the accelerometer are directed to three of 10-bit analog-to-digital converter (ADC) units on the XBee module, shown in Figure 3(c) The corresponding digital data is sent to the OEM RF module providing wireless data communication over ZigBee/802.15.4 protocol [9] Hence, XBee modules operate in the license-free 2.4 GHz ISM band with a RF data rate of 250 kbps

With the receiver sensitivity of í97 dBm, the typical transmission range of an Xbee module can be adjusted for 30–120 meters Using XBee 802.15.4 the transmit power output can be boosted as high as

60 mW (+18 dBm) Each XBee module comes in either a regular or long-range “–PRO” version In our design we used the PRO version to ensure maximum penetration through the shielded wall

The Xbee module can be configured using a series of AT commands More advanced features, such

as supporting multi-node topology, data transmission, data reception, etc, can be configured using APIs (application programming interface) The configuration can be performed locally or over the air

In our design we implemented point-to-point configuration between WN and BS The wireless node is powered by two AA batteries at about 3.3 V For the best performance, it is important to use fully charged batteries

3.2 Base Station (BS)

The base station consists of an Xbee transceiver connected to a PC The Xbee transceiver directs the received digital data from the wireless node and passes it onto the PC via a USB interface Using our own developed software, the received data is analyzed and displayed As shown in Figure 3(a), the BS

can be interfaced to the linear accelerometer and an audible alarm can notify technicians of any undesired patient change of orientation

An important consideration in configuring the BS is properly setting its sensitivity level The receiver sensitivity of XBee can be set to as low as í100 dBm The following expression shows the relation between the receiver sensitivity, ɎdBm , in dBm and the transmitting power of the WN:

)], ( ) (

[ log[

10 P tx o d1 d2 sh d3

where:

2 2

1 2

1 ) [4 ( ) / ]

(2)

In the above expressions Ptx is WN’s transmitted power in mW, Dsh is the attenuation factor of the shielded wall as a function of its thickness (d3) defined in [10], and Do is the free-space attenuation with d1 and d2 being the distance between the WN and BS, shown in Figure 2(a) In Equation (2), C is the speed of light in free-space and f represents the license-free 2.4 GHz ISM band.

4 Signal Processing Algorithm

The received data from the accelerometer must be properly filtered and interpreted in order to accurately measure patient’s orientation The signal processing algorithm constantly samples the accelerometer for static tilt information

Figure 4 depicts the flowchart for the signal processing algorithm Initially, the received data from

the accelerometer must be converted to tilt angle in order to properly determine the head rotation around X, Y, Z axes Hence, the measured values from the accelerometer is compared to the zero g offset to determine if it is a positive or negative acceleration, e.g., if value is greater than the offset then

the acceleration will indicate a positive acceleration, so the offset is subtracted from the value and the resulting value is passed to a tilt equation to determine the corresponding degree of tilt

The following expressions are used to determine the tilt angle:

M

Tdeg sin1(V measuredV offset)

or:

M

S

180

rad

V

In the above expressions Vmeasured is the accelerometer readout sampled by the ADC channel in mV Furthermore, ij represents accelerometer’s sensitivity in mV/g The value of Voffset indicates the

accelerometer’s zero g offset In our application, value of V offset for each axis was found independently Note that the sign of T determines the direction of the tilt (e.g., up/down or left/right) Using Equation

(3), when the tilt angle is zero degrees (zero g), V measured = Voffset The sensitivity value can be obtained from taking the difference between the measured values from the accelerometer at zero and 90 degrees, corresponding to zero and one g Hence, the sensitivity can be calculated as follows:

) 0 ( )

1

'

'

g V

g V

g

V

measuredt measured

Using the above expressions, with a measured sensitivity value of 270 mV/g and having 10-bit ADC embedded in the Xbee transceiver powered by a 3.3 V power supply, it can be demonstrated that APOMS can accurately offer 0.23 degree resolution at zero degree Lower resolution is expected as the value of g increases

As an example, consider the case where patient’s head rotates along X-axis (tilting up or down), as shown in Figure 2(b) For X-axis, the sensitivity, M , has been measured to be 800 mV/g and

Voffset (g = 0) is determined to be 1,650 mV After an upward head tilt, assuming the accelerometer reads 1,750 mV, we can calculate the tilt angle of the head around X-axes to be about 7.2 degrees This indicates that the patient’s head rotated 7.2 degrees upwards compared to its initial coordinates

The received raw data from WN is sampled at a rate of 200 Hz This sampling rate is sufficient to detect any change in orientation of the patient’s head However, the accelerometer is susceptible to noise In order to ensure filtering the erroneous values measured by the accelerometer, the signal processing algorithm computes a running average to record the mean measured raw data over a period

of one second The choice of one second time interval was tested to be a good compromise between accurately obtaining data, while quickly detecting any changes in orientation The latest average tilt value is calculated using the following expression:

s

i s

i i s

)

1





where s is the data sampling rate (200 samples per second) In the above equation, Ts and T1 represent the latest and the first recorded (displayed) tilt angle, respectively, in the running average window The running root-mean-square (rms) average of Ts for all three axes is also computed using:

2 2

) ( )

As shown in Figure 4, each average tilt value, including the rms average, is compared with the previous value to detect any change in orientation Additional tolerance limits,Tth, can be assigned to each axis for allowing a small acceptable tilt Consequently, having Ts !Ts1Tth indicates the patient rotating in the positive direction, where as havingTs Ts1Tth, represents rotation in the negative direction Under either condition, the interrupt circuitry and/or the alarm can be activated

5 Software Implementation

Various software interfaces can be implemented to communicate with the Xbee module and process the signals from the accelerometer In our prototype, we used LabVIEW programming language [11] for its ease of programming and versatility in data acquisition LabVIEW is a graphical program development application developed by National Instruments LabVIEW program consists of two parts: the control panel, providing the graphical user interface (GUI) and the block diagram, containing all the programming codes

Figure 4 Flowchart of the signal processing algorithm used in APOMS

Figure 5 shows the control panel designed for the APOMS In this figure we only depict X- and

Y-axis monitors due to space limitation

Figure 5 Control panel of APOMS using LabVIEW platform monitoring rotations about

(a) X and Y axes and (b) Z axis

(a) (b)

Upon pressing the Lock button on the control panel, the user enables a reference orientation about X,

Y, Z axes (e.g.,To (x)) desired for radiation For each independent axis a (+/í) tolerance limit can be assigned, allowing the patient limited flexibility in rotation when undergoing radiation therapy If the patient rotates beyond either one of the tolerance limits, To(x)rTth(x), the alarm indicator button

Radiation in Progress will be activated and an optional audible alarm can be set off The selection of

the tolerance limits for each axis depends on the accuracy of the dosimetric and radiation therapy procedure

Figure 5 also shows the directional indicators on the control panel, identifying the direction of the patient’s head rotation (e.g., right/left and up/down) This information can be used for adjusting the patient’s head to the reference coordinates

The GUI-based APOMS control panel offers a number of advanced features that can be enabled We briefly mention a few of these features in the following paragraphs We note that these features have not gone under any clinical testing and future studies are required to qualify their efficiency and practicality

Record keeping: Using this feature all changes in orientation can be accurately time stamped and

recorded in a database, while the patient is undergoing radiation therapy Study of such information can potentially allow more precise dosimetric plans for individual patients

Automatic reorientation: By interfacing the APOMS software to external LED panels, it is possible

to notify the patient of any excessive head rotation Furthermore, APOMS can guide the patient to shift

to the reference coordinates through a series of audible commands Clearly, the radiotherapy must be halted while the patient is being redirected to the reference coordinates

Radiation interruption: The APOMS software can easily be interfaced with the linac Hence, once

the patient’s orientation is changed beyond the defined tolerance limits, the system can interrupt the radiation The linac interrupt circuitry can be activated via wired or wireless communications Immediate interruption of the radiation can avoid exposure of healthy tissues and unnecessary dose

External synchronization: The APOMS design is capable of supporting additional sensors APOMS

can also receive timing information from external devices Hence, APOMS can synchronize with external devices and assist skin shift monitoring

Independent tolerance limits: Separate tolerance limits for rotations about each independent axis

(e.g., +X or –X) can be enabled in the APOMS software This can allow more flexible dosimetric planning for patients

6 Experimental Setup and Results

In order to characterize the accelerometer, we mounted the WN on a Flexiholder [12], as shown in Figure 6 Flexiholder is capable of moving a small platform in three dimensions Rotation about each individual axis was tested independently The data sampling rate and the moving average filter were set to 200 Hz and one sample per second, respectively Each experiment was repeated multiple times

to guarantee that the mean results a confidence interval of 15% or better at 90% confidence level All reported results are referenced to X-, Y-, Z-axis of the accelerometer, as shown in Figure 3(b)

Figure 6 The wireless accelerometer was mounted on the Flexiholder from Huestis Medical

Figure 7(a) shows the typical APOMS voltage readouts for three different patients undergoing

radiation therapy for about three minutes This figure indicates that none of the patients changed

his/her head orientation during the period of radiation Figure 7(b) depicts a typical readout of

APOMS, while monitoring a patient for a brief period of time after the system is locked The figure

shows that the patient inadvertently tilted her head from the reference (locked) coordinates Figure 8

shows T(xyz) rms for the same scenario Note prior to the sudden head tilt, initial changes in orientation

were within the tolerance limits When the change in orientation exceeds the threshold (e.g., To rTth)

the alarm is activated

Figure 7 APOMS reading for all three axes: (a) typical voltage readouts for three different

patients undergoing radiation for about three minutes; (b) patient tilting her head

300

350

400

450

500

34133340 34193340 34253340 34313340

Time (msec)

Patient 1 Patient 2 Patient 3

x

z

z

x

y

x

y

z

400 420 440 460 480 500 520 540 560

Time (msec)

X-rotation Y-rotation Z-rotation

(a) (b)

The sensitivity and repeatability of APOMS in X-axis are demonstrated in Figure 9 Similar results

can be achieved for Y- and Z-axis For practical purposes, we only tested the changes within 15

degrees The Min and Max values represent the lowest and highest values recorded These results

show that as the tilt angle increases, slightly larger variations from the mean are observed This is due

to the nonlinearity of the accelerometer’s output Hence, the accelerometer is most sensitive when the

sensing axis is closer to zero degrees (zero g), and less sensitive when closer to 90 degrees (one g)